Estimates of the prevalence of HPV infection have been based on the detection of HPV DNA at genital sites, and this approach has not allowed a distinction between primary or recurrent infection. The ability to use HPV antibody positivity as a marker of infection has been hampered by the lack of a suitable source of HPV antigens. Our initial efforts using bacterially expressed proteins in Western blots provided firm evidence of HPV serum antibodies but suggested that antibodies reactive with conformational epitopes may be an important component of the immune response. The recent development in our laboratory of vaccinia virus expressed capsids that are structurally identical to authentic HPV virions has allowed the development of ELISAs that detect "type specific" antibodies reactive with a conformational epitope(s) on the major capsid protein, L1. Seropositivity to HPV 1 capsids among women reporting a history of foot warts was 89 percent. Seropositivity to HPV 6 was associated with current or historical genital warts in several populations, with HPV 6/11 DNA detected in the genital tract and with number of sexual partners in a population of college women. These results form the basis for proposing to analyze sera for antibodies to HPV types 6 and 16 in two ongoing cohorts: college women who are initiating sexual activity and a cohort of pregnant women and their babies. The longitudinal collection of sera will allow us to study the kinetics of appearance and disappearance of anti-capsid antibodies, to provide evidence of seroconversion, to examine the appearance of IgM and IgG antibodies and to define the parameters that are associated with the development of an antibody response such as viral load, viral type, age, gender, sequelae of infection and other factors. The L1 gene will be cloned, sequenced and expressed from subjects who are seronegative but DNA positive, to determine whether antigenic variants can explain the lack of seroreactivity. Mucosal secretions will be obtained to look for evidence of local antibodies. Phylogenetic analyses have provided insights into the relatedness of HPV types. Vaccinia virus recombinants expressing the major capsid proteins for all of the genital types and/or other phylogenetically related types, e.g. 2 and 27 that have only been detected at cutaneous sites, will be constructed and the capsids will be used to determine whether a system of HPV serotypes can be developed. Cross-reactivity will be assessed using experimental derived sera and in blocking experiments with human sera. As new capsids are developed, - seroprevalence to these types will be determined. These data will be an important adjunct for understanding the natural history of HPV infection and will provide critical information for the development and testing of HPV vaccines.